1. Field of the Invention
This invention relates to multi-terminal computer systems employing redundant communication links between the computer and the terminals to allow continued operation in the event of a malfunction as well as diagnostic tests to determine the condition of the system elements.
2. Prior Art
Systems employing a central computer connected by a communication channel to a plurality of remotely located terminals are employed in a variety of applications. They may be used in factories to collect and distribute production information and instructions; provide the facilities of the central computer to a variety of users, and in many other applications. The terminals may constitute simple input or output terminals or they may include their own computers which perform various tasks and exchange data with the head-end computer over the communication channel.
Often these systems have polling capability allowing the head-end computer to interrogate the terminals in rapid order to determine their status and to collect information from them. U.S. Pat. No. 3,821,706, assigned to the assignee of the present invention, discloses a system of this type in which each terminal has a unique address and a counter in the terminal is advanced each time the computer outputs a digital command to the system. When the status of the counter in a terminal coincides with the terminal's address, the terminal transmits its status onto the communication channel and the central computer determines the origin of the message based on the status of its counter.
When these systems are employed in applications where substantial damage or economic loss may occur if any part of the system fails, it becomes desirable and economical to employ redundant system elements connected to maintain system operation despite certain types of failure. For example, systems have been provided in which a pair of independent signal transmission lines extend between the head-end computer and each of the terminals. Separate transmitters are provided for each of these lines. The outputs of two lines are summed at each terminal. Only one of the transmitters is used at any time and if failure of the energized transmitter, or some portion of the line connecting it to each of the terminals, is noted, the system switches to the other transmitter and its communication line, preserving operation of the system. This system may be economical when all of the terminals are relatively closely spaced to the central computer, but in applications employing widely spaced terminals the added cost of the redundant transmission line and its installation may be prohibitive.